PERSISTENCE OF RUST RESISTANCE 567 



SCHREINER: Hans, maybe I didn't quite follow you, but why do you 

 think that field resistance must be reinforced with some type of inocula- 

 tion research? Don't you think field resistance is a sufficiently good 

 measure? 



HATTEMER: I indicated, that in the field there is the risk of 

 escapes. Trees we regard as resistant really are not. The second point 

 is that we never have complete control over the genotype of the inoculum. 

 It is virtually impossible in the field to figure out with which races 

 our material was infected. It's real difficult to predict whether there 

 exists something like geographic variation in Cvonavtivm ribicola as it 

 does seem to exist in other fungi which weren't introduced into this 

 country. I mean we need a mutual backing up of the two test procedures. 



BINGHAM: I would like to try to answer Roger Blair's question as 

 I see it. Geral McDonald has helped us make some calculations of 

 inbreeding coefficients in large full-sib families. Our calculations show 

 very low values. For 10 families, the inbreeding was 2 percent in three 

 generations or something like that. This certainly isn't much to worry 

 about, Roger. I think what you should worry about is if you try to work 

 within a confining population of this size, then you're hopelessly con- 

 fined for variability in other traits. 



BORLAUG: I would like to add one comment to this discussion that's 

 been taking place the last few seconds. You narrow the base too much, and 

 you lay yourself very vulnerable to secondary diseases that might have 

 been of no significance in the past. Your genetic base has been greatly 

 restricted, and you haven't paid any consideration whatsoever to this. 

 I know that our maize {Zea mays) breeder, Ed \\ellhausen, would die if 

 he heard you talking about taking three individual plants out of a maize 

 population and starting to propagate them, or use them as a base for 

 breeding rust resistance. We would not worry about the rust, but some of 

 the other diseases that are of no significance in populations as they 

 exist now. 



WEISSENBERG: It seems to me that the white pine resistance breeders 

 now are in a situation where they will try to breed for a host population 

 similar to the populations of P. peuoe and P. sylvestvis with respect to 

 their associated rusts, C. ribicola and Pevidevmiwn pini. Here are two 

 models. A system is functioning where the host and the pathogen are in 

 some kind of equilibrium. I would like to hear Dr. Hattemer's comment 

 on what we can learn from these systems and how to go about studying these 

 systems. These types of systems seem to be the goal for the white pine 

 breeders in North America, in other words, to achieve the model Mother 

 Nature provides us with. 



HATTEMER: I wouldn't give you any recipe on how many generations it 

 takes for something to originate as it exists in the host-pathogen rela- 

 tionships that you were talking about. I have no comment whatsoever on 

 this. 



SCHREINER: On this matter of a balanced population, I would remind 

 you that an introduced pest wiped out our native chestnut; there was never 

 time to set up such bounds. 



BORLAUG: I think this is a very good point. We don't live very long, 

 and the way things are going now, time is pretty short. 



